Barbituric and thiobarbituric acid and their derivatives have been used as components of redox initiating systems in polymerizable compositions. In particular, barbituric acid in combination with a halogen compound and a metal ion has been found to be efficient in curing methacrylates. For example, a polymer initiating system containing a barbituric or thiobarbituric acid compound and a copper or ferrum halogenide has been used for curing methacrylate compositions. An initiating system using a barbituric acid derivative and copper (II) chloride has also been used. An initiating system comprising barbituric or thiobarbituric acid derivative, a copper salt, and a sulfinic acid compound has been used for curing unsaturated, acidic resins. However, conventional initiating systems that use barbituric acid, thiobarbituric acid, and/or their derivatives as part of redox initiating system generally do not provide shelf-life stable compositions in contact with unsaturated compounds, such as, e.g., methacrylates. Thus, there is a need to have an improved polymerizable composition with enhanced shelf-life stability.